1. Field of the Invention
The present invention relates generally to the fields of optical measurements. More particularly, it concerns methods and systems for measuring spatially-resolved scattering of an optical medium, including the eye.
2. Description of Related Art
Because optical media, including the eye, may include several types of defects that negatively affect image quality, several instruments have been developed in an attempt to measure and correct such defects. One such instrument is the spatially-resolved optometer. The idea of a spatially-resolved optometer originated in the early 19th century with Thomas Young, who employed a double pinhole technique to sample a wavefront entering the eye at two locations. Using such an instrument, one may obtain a gross measure of the refractive state of the eye, but one may not discriminate between defocus, astigmatism, and higher order aberrations. Also, the Thomas Young optometer requires subjective judgments by the observer, which introduce sources of uncontrolled variability. Modern clinical optometers (e.g. auto refractors) have the advantage of objectivity, but still suffer from limitations similar to those suffered by the optometer of Thomas Young.
Recent technological advances in the field of visual optics research have made it possible to quickly obtain an objective, high-resolution map of the refractive state of the eye at several hundred sample points within the eye's pupil. From this matrix of sample points, it is possible to derive a wavefront aberration function of the eye, which is a fundamental description of the eye's optical characteristics from which traditional measures of refractive error (e.g. the degree of myopia, hyperopia, or astigmatism) may be derived. Furthermore, the wavefront aberration function may be used to compute metrics of retinal image quality, such as the point spread function or the optical transfer function. For at least these reasons, the wavefront aberration function is a richer and more useful description of the optical properties of the eye than is obtained by conventional optometers.
Currently available methods for measuring the eye's wavefront aberration functions fall into two broad categories—subjective and objective. However, each of these methods share a common principle: the eye's pupil is sub-divided into a number of sub-apertures, the rays passing through these sub-apertures are isolated, and their direction of propagation measured. Aberrations are then quantified by the deviation of these isolated rays from the trace of corresponding rays in an aberration-free system. In effect, a beam of light passing through the eye's pupil is spatially resolved into a number of smaller beams which are independently measured in order to gain detailed characterization of the refractive anomalies of an eye.
Subjective methods for measuring ray aberrations all require that the subject judge the apparent visual direction of discrete points in the retinal image that result from rays passing through specific points in the pupil plane. In the aberroscope method, for example, a grid is placed over the pupil, and the eye is deliberately defocused in order to produce a blur circle that replicates the shape of the pupil, including the grid of opaque lines. Since each intersection of grid lines corresponds to a specific pupil location, distortions seen in the retinal shadows of the grid may be used to infer the directions of identified rays as they leave the eye pupil and eventually intersect the retina. In effect, the subjective aberroscope is a ray tracing device that allows the simultaneous monitoring of multiple rays as they pass through known pupil locations. The subject's task in this case is to estimate the apparent visual direction of each point on the grid by perceptually interpreting the retinal stimulus as if it were the conventional image of a real object formed by the eye's optical system.
An alternative subjective approach developed originally by Smirnov uses a single pinhole aperture to isolate a narrow bundle of rays from an axial point source as they pass through a known location in the eye's pupil. In an aberrated eye, the retinal intersection of this ray bundle will not coincide with the axial retinal image of a reference point source. The subject will therefore perceive the test and reference point sources as having different visual directions. To nullify this difference in visual direction, the subject displaces the test light until it appears to coincide with the reference light. In this way, the ray aberration is transferred from image space to object space where it may be measured quantitatively. Unfortunately, the method of subjective magnitude estimation is inherently unreliable, and the nulling method used in the Smirnov technique is very time consuming. In short, both methods place an excessive demand on the subject, which makes both techniques unsuitable for routine use with patients in a clinical setting.
In response to the problems of subjective techniques, certain objective methods for measuring wavefront aberration functions of the eye have been developed. One method is an objective aberroscope which uses a fundus camera to record an image of a distorted grid on the retina. One of the limitations of the objective aberroscope is that the eye's optical system serves as the objective lens for the fundus camera. Consequently, even for eyes with normal levels of aberration, the eye's imaging quality limits the minimum useable spacing between grid lines in the pupil plane to about one millimeter, thus limiting the resolution with which the eye's aberrations may be specified. For the same reason, a highly aberrated eye may not yield an image of sufficient quality to allow reliable measurements of the aberration function with such a device.
Both of these problems with the aberroscope method are avoided by another objective method, the Hartmann-Shack wavefront sensor, which characterizes the eye's aberration in object space. Developed for astronomical applications and adapted recently for the eye, this wavefront sensor uses an array of lenses and a video detector to measure rays of reflected light emerging from the eye. In the Hartmann-Shack method, a narrow beam of light is directed into the eye to produce a high quality point source of light on the fundus. Light reflected from the fundus is then subdivided into a large number of ray bundles as it leaves the eye. This is achieved by placing an array of tiny lenses in a plane conjugate with the eye's pupil plane. A video detector (CCD) located at the focal plane of the lenslet array thus records an array of point images, one for each lens in the array. For an eye free of defocus and aberrations, all of the exiting ray bundles would be parallel, and therefore the CCD would record an array of point images with the same imagery as the lenslet ray. Deviations from this geometry may therefore be attributed to aberrations in the exiting beam. Given that high density lenslet arrays are now available commercially from, e.g., Adaptive Optics Associates (Cambridge, Mass.), the Hartmann-Shack method for measuring the wave aberration function in human eyes is emerging as an important method for fast, objective high spatial resolution in the pupil plane.
Although the above techniques have exhibited at least a degree of usefulness for determining spatially-resolved aberration functions of the eye, those methods ignore information relating to localized scatter (and absorption) within the eye. In particular, current methods utilizing Hartmann-Shack measurements concentrate upon the deviation of spot images within a Hartmann-Shack image from that of an ideal geometrical array rather than concentrating upon scattering and absorption information present within those images. Because a wide variety of optical imperfections may cause scattering of light, absorption of light, or aberrations too fine to be resolved by a wavefront sensor, it would be advantageous to provide for the ability to measure local scattering and absorption within an optical medium, including the eye. More particularly, abnormalities including, but not limited to, abnormalities of the tear film, corneal scars, opacities, vacuoles, edema, foreign bodies in the anterior or vitreal chambers, cataracts, gradients in refractive index, structural abnormalities of the crystalline lens, drusen, pigmentation of the neural retina, or optical dirty contact lenses or spectacle lenses may all serve as a source of scatter and/or absorption that may be identified and/or localized if one has the ability to effectively measure local scatter and/or absorption within a media.